The present invention relates generally to an apparatus and method for the removal of entrained liquid from vapor streams. More specifically, the present invention relates to two-stage, chevron vane-type mist eliminators and methods of using same to remove liquid droplets from vapor streams.
Mist eliminators are commonly used in vessels such as distillation towers, knockout drums, or other process apparatuses to remove entrained or suspended liquid droplets or mist from vapor streams flowing within the vessels. Mist eliminators can be employed in a wide variety of vapor-liquid separation applications. For example, they are used to remove pollutants or other contaminants from flue gas streams and to separate a desirable liquid from a vapor stream in a distillation column.
Mist eliminators conventionally take a variety of forms such as rod banks, mesh pads, and vanes. Vane-type mist eliminators employ a plurality of plates or blades that are positioned in spaced-apart and side-by-side relationship to form multiple vapor flow paths in the spacing between adjacent blades. The blades typically are identically constructed and have multiple angled surfaces, such as a chevron-profile, that create repeated tight bends in the vapor flow paths. As the vapor stream navigates these bends, the inertia or momentum of the liquid droplets in the vapor stream causes the liquid droplets to impinge against and adhere to the blade surfaces. The small droplets then coalesce into larger droplets that then drain downwardly along the blades under the influence of gravity. In this manner, some portion of the liquid droplets is removed from the vapor stream.
In applications where the vapor stream carries a heavy liquid load or where a high degree of liquid removal from the vapor stream is desired, it is known to use a two-stage vane-type mist eliminator to increase the liquid-removal capacity of the mist eliminator. In one embodiment of such a two-stage mist eliminator, the upstream stage is linear and extends horizontally or perpendicular to the direction of vapor stream flow. The blades in the downstream stage are arranged to form a V-shaped profile having an apex pointing in the direction of the vapor stream flow. The resulting triangular-shaped configuration is advantageous in that the linear upstream stage removes a portion of the liquid droplets before the vapor stream encounters the V-shaped downstream stage, thereby increasing the capacity and efficiency of the mist eliminator in comparison to many single-stage mist eliminators. The sloping profile of the V-shaped downstream stage is also advantageous in that it allows coalesced liquid to readily drain from the blades by following the slope of the blades. This advantage, however, is somewhat offset by the relatively less efficient drainage of liquid from the horizontally oriented blades in the underlying upstream stage of the mist eliminator, which can result in flooding of the upstream stage and reentrainment of the coalesced liquid into the vapor stream. The capacity and efficiency of this type of two-stage mist eliminator is thus limited by the performance of the horizontally oriented upstream stage.
Examples of two-stage mist eliminators which partially overcome the disadvantages of the triangular two-stage mist eliminators discussed above are disclosed in U.S. Pat. No. 5,749,930. In that patent, the blades in each stage are arranged to form identical V-shaped profiles. In one embodiment, the stages are arranged so that the apexes of their V-shaped profiles point toward each other. In another disclosed embodiment, the apexes point away from each other. This latter embodiment is advantageous because it allows the paired stages to be mounted to common beams. In yet another embodiment described as being in the prior art, the apexes of both stages point in the direction of vapor flow.
A disadvantage common to the embodiments disclosed in U.S. Pat. No. 5,749,930 is they require a greater vertical height than the triangular design discussed above. Another disadvantage common to the embodiments in which the apex of either stage points in the direction opposite the vapor flow direction is the liquid drains toward the apex into the middle of the vapor stream where it is more likely to become reentrained in the vapor stream. In addition, in the embodiment in which the apexes point away from each other, liquid that drains from the overlying downstream stage onto the underlying upstream stage will then drain toward the apex of the upstream stage, thereby further reducing the capacity and efficiency of the upstream stage. This embodiment, however, has an advantage in that the lateral sides of the two stages are positioned closely together and are supported on a common pair of beams. The lateral sides of the two stages in the other embodiments are positioned a greater distance apart and are shown as being supported on separate support structures.
A three-stage mist eliminator is disclosed in German Patent Publication No. 20 2005 002 674 U1 and uses a bank of rods as the first stage and blades in the second and third stages. The blades in the second and third stages differ in type and are arranged in a triangular configuration. Because the second stage extends linearly, it suffers the same drainage problems of the triangular designs discussed above. The use of three stages is also disadvantageous because it increases the height of the mist eliminator in comparison to the two-stage mist eliminators discussed above.
A need thus exists for a two-stage mist eliminator in which the two stages are supported on a common support structure and in which the capacity and efficiency of the upstream stage is improved over the designs described above in which the upstream stage is linear or has an apex pointing in the opposite direction of the vapor stream flow.